The transportation of liquid commodities by rail, specifically by tank cars, is a vitally important portion of all of the North American economies. While all tank cars are capable of some degree of pressure retention, certain types are specifically designated as “pressure” cars. These cars are designed for the transportation of liquefied gasses, such as liquid propane gas (LPG), anhydrous ammonia, and chlorine, among many others and are subject to stringent control with regard to design, inspection, qualification, and operation of the vehicle. Furthermore, many of the commodities transported in tank cars are classified as hazardous by the Federal Railroad Administration (FRA). Any car, pressure or non-pressure, which transports hazardous commodities, must comply with the Department of Transportation (DOT) specification approved for that commodity.
The Association of American Railroads (AAR) augments the FRA rules with additional requirements for certain commodities as well as tank cars in general. For example, while the DOT federal regulations specify the materials of construction for tank cars, they do not require that the steels used for the construction of pressure cars be normalized. The AAR, however, has set forth its own requirements in addition to those of the DOT, that such steels be in the normalized condition. The purpose of these additional requirements is to enhance the ability of the tank cars to resist fracture when loaded beyond normal conditions encountered in the course of transportation, such as in accidents.
As the quantity of hazardous materials shipped by rail increases over time, there is ever greater attention paid, and regulations directed, to the improvement of the ability of tank cars to resist fracture in accident situations. Both the design of the tank car and the materials of construction have been subject to review. The general direction of improvements has been towards thicker materials and welds having an enhanced ability to resist brittle fracture, that is, increased toughness.
The prior art welding of tank car tanks consists of single or multiple wire submerged arc welding (SAW) processes. The SAW process consists of a weld wire through which the welding current is passed, causing the formation of the welding arc and melting of the wire to make the weld deposit. A granular flux is deposited over the welding arc, causing the arc to be submerged beneath. This process has a heat input, measured in Joules/inch, that is sufficiently high to obtain both good penetration into the base metal and high deposition rates. The high heat input, however, can be deleterious to the toughness of the weld deposit, and in spite of the high deposition rate, is slow. In addition, a great deal of heat input (energy) is wasted due to the melting of large volumes of base plate material not necessary for the formation of the weld.
Welding thicker sections and providing lower heat inputs (for greater toughness) requires multiple passes when using the SAW process, since less welding metal is deposited per pass. As a result, more passes are required to fill the joint with welding metal. This causes the tank car building times and costs to increase substantially due to higher labor costs. The necessity of cleaning fused flux between passes increases cost and time even more.
The hybrid laser arc welding (HLAW) process has been under development for some time, but has yet to find use in heavy plate welding requiring circular girth seams as is found in railroad tank cars. Furthermore, the application of HLAW for railroad tank cars has some additional special requirements not found in other industries: 1) tank cars are constructed of individual shell courses, each approximately ten feet wide, which are rolled into shell sections which vary from ninety inches inside diameter to as much as one hundred and nineteen inches in diameter; 2) the thickness of tank cars varies from seven-sixteenths of an inch to one inch or more; 3) some tank cars, including those carrying liquefied gasses under pressure, are required to meet stringent toughness requirements for the welds and heat affected zones (these requirements are expected to be extended to most non-pressure cars in the near future); 4) the individual shell courses are butted together to make girth seams which must be welded in the roundabout position and 5) there are potential changes to the regulations being considered by the Association of American Railroads and the Department of Transportation which, if enacted, would expand the new car construction requirement for shell and weld toughness to most, and probably all, DOT class tank cars.
Therefore, a need exists for a system and method for welding tank car tanks that has the capability to weld all thicknesses in a single pass, is simple to switch from one thickness to another, and yet has a low heat input for meeting the toughness requirements for tank car construction.